wpf ComboBox的SelectionBoxItem相关依赖属性

以前没有注意SelectionBoxItem相关依赖属性，这几天看wpf源码 特意研究了一番

<Style x:Key="ComboBoxStyle1" TargetType="{x:Type ComboBox}">
            <Setter Property="FocusVisualStyle" Value="{StaticResource ComboBoxFocusVisual}"/>
            <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.WindowTextBrushKey}}"/>
            <Setter Property="Background" Value="{StaticResource ButtonNormalBackground}"/>
            <Setter Property="BorderBrush" Value="{StaticResource ButtonNormalBorder}"/>
            <Setter Property="BorderThickness" Value="1"/>
            <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
            <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
            <Setter Property="Padding" Value="4,3"/>
            <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
            <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
            <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type ComboBox}">
                        <Grid x:Name="MainGrid" SnapsToDevicePixels="true">
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition Width="*"/>
                                <ColumnDefinition MinWidth="{DynamicResource {x:Static SystemParameters.VerticalScrollBarWidthKey}}" Width="0"/>
                            </Grid.ColumnDefinitions>
                            <Popup x:Name="PART_Popup" AllowsTransparency="true" Grid.ColumnSpan="2" IsOpen="{Binding IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Margin="1" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Placement="Bottom">
                                <Microsoft_Windows_Themes:SystemDropShadowChrome x:Name="Shdw" Color="Transparent" MaxHeight="{TemplateBinding MaxDropDownHeight}" MinWidth="{Binding ActualWidth, ElementName=MainGrid}">
                                    <Border x:Name="DropDownBorder" BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}" BorderThickness="1" Background="{DynamicResource {x:Static SystemColors.WindowBrushKey}}">
                                        <ScrollViewer x:Name="DropDownScrollViewer">
                                            <Grid RenderOptions.ClearTypeHint="Enabled">
                                                <Canvas HorizontalAlignment="Left" Height="0" VerticalAlignment="Top" Width="0">
                                                    <Rectangle x:Name="OpaqueRect" Fill="{Binding Background, ElementName=DropDownBorder}" Height="{Binding ActualHeight, ElementName=DropDownBorder}" Width="{Binding ActualWidth, ElementName=DropDownBorder}"/>
                                                </Canvas>
                                                <ItemsPresenter x:Name="ItemsPresenter" KeyboardNavigation.DirectionalNavigation="Contained" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
                                            </Grid>
                                        </ScrollViewer>
                                    </Border>
                                </Microsoft_Windows_Themes:SystemDropShadowChrome>
                            </Popup>
                            <ToggleButton BorderBrush="{TemplateBinding BorderBrush}" Background="{TemplateBinding Background}" Grid.ColumnSpan="2" IsChecked="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}" Style="{StaticResource ComboBoxReadonlyToggleButton}"/>
                            <ContentPresenter ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" IsHitTestVisible="false" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                        </Grid>
                        <ControlTemplate.Triggers>
                            <Trigger Property="HasDropShadow" SourceName="PART_Popup" Value="true">
                                <Setter Property="Margin" TargetName="Shdw" Value="0,0,5,5"/>
                                <Setter Property="Color" TargetName="Shdw" Value="#71000000"/>
                            </Trigger>
                            <Trigger Property="HasItems" Value="false">
                                <Setter Property="Height" TargetName="DropDownBorder" Value="95"/>
                            </Trigger>
                            <Trigger Property="IsEnabled" Value="false">
                                <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.GrayTextBrushKey}}"/>
                                <Setter Property="Background" Value="#FFF4F4F4"/>
                            </Trigger>
                            <Trigger Property="IsGrouping" Value="true">
                                <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
                            </Trigger>
                            <Trigger Property="ScrollViewer.CanContentScroll" SourceName="DropDownScrollViewer" Value="false">
                                <Setter Property="Canvas.Top" TargetName="OpaqueRect" Value="{Binding VerticalOffset, ElementName=DropDownScrollViewer}"/>
                                <Setter Property="Canvas.Left" TargetName="OpaqueRect" Value="{Binding HorizontalOffset, ElementName=DropDownScrollViewer}"/>
                            </Trigger>
                        </ControlTemplate.Triggers>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
            <Style.Triggers>
                <Trigger Property="IsEditable" Value="true">
                    <Setter Property="BorderBrush" Value="{StaticResource TextBoxBorder}"/>
                    <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.WindowBrushKey}}"/>
                    <Setter Property="IsTabStop" Value="false"/>
                    <Setter Property="Padding" Value="3"/>
                    <Setter Property="Template" Value="{StaticResource ComboBoxEditableTemplate}"/>
                </Trigger>
            </Style.Triggers>
        </Style>

可以看到在文本模式下 ContentPresenter 相关依赖属性 ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" Content="{TemplateBinding SelectionBoxItem}" ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}"是通过这几个SelectionBoxItem属性绑定的

以下为ComboBox相关源码

通过获取InternalSelectedItem为ComboBoxItem的content,contentTemplete,contentStringFormat属性传递给internalSelectedItem，itemTemplate ，itemStringFormat

 private void Update()
        {
            if (this.IsEditable)
            {
                this.UpdateEditableTextBox();
            }
            else
            {
                this.UpdateSelectionBoxItem();
            }
        }

private void UpdateSelectionBoxItem()
        {
            object internalSelectedItem = base.InternalSelectedItem;
            DataTemplate itemTemplate = base.ItemTemplate;
            string itemStringFormat = base.ItemStringFormat;
            ContentControl control = internalSelectedItem as ContentControl;
            if (control != null)
            {
                internalSelectedItem = control.Content;
                itemTemplate = control.ContentTemplate;
                itemStringFormat = control.ContentStringFormat;
            }
            if (this._clonedElement != null)
            {
                this._clonedElement.LayoutUpdated -= new EventHandler(this.CloneLayoutUpdated);
                this._clonedElement = null;
            }
            if (((itemTemplate == null) && (base.ItemTemplateSelector == null)) && (itemStringFormat == null))
            {
                DependencyObject d = internalSelectedItem as DependencyObject;
                if (d != null)
                {
                    this._clonedElement = d as UIElement;
                    if (this._clonedElement != null)
                    {
                        VisualBrush brush = new VisualBrush(this._clonedElement) {
                            Stretch = Stretch.None,
                            ViewboxUnits = BrushMappingMode.Absolute,
                            Viewbox = new Rect(this._clonedElement.RenderSize),
                            ViewportUnits = BrushMappingMode.Absolute,
                            Viewport = new Rect(this._clonedElement.RenderSize)
                        };
                        DependencyObject parent = VisualTreeHelper.GetParent(this._clonedElement);
                        FlowDirection direction = (parent == null) ? FlowDirection.LeftToRight : ((FlowDirection) parent.GetValue(FrameworkElement.FlowDirectionProperty));
                        if (base.FlowDirection != direction)
                        {
                            brush.Transform = new MatrixTransform(new Matrix(-1.0, 0.0, 0.0, 1.0, this._clonedElement.RenderSize.Width, 0.0));
                        }
                        Rectangle rectangle = new Rectangle {
                            Fill = brush,
                            Width = this._clonedElement.RenderSize.Width,
                            Height = this._clonedElement.RenderSize.Height
                        };
                        this._clonedElement.LayoutUpdated += new EventHandler(this.CloneLayoutUpdated);
                        internalSelectedItem = rectangle;
                        itemTemplate = null;
                    }
                    else
                    {
                        internalSelectedItem = ExtractString(d);
                        itemTemplate = ContentPresenter.StringContentTemplate;
                    }
                }
            }
            if (internalSelectedItem == null)
            {
                internalSelectedItem = string.Empty;
                itemTemplate = ContentPresenter.StringContentTemplate;
            }
            this.SelectionBoxItem = internalSelectedItem;
            this.SelectionBoxItemTemplate = itemTemplate;
            this.SelectionBoxItemStringFormat = itemStringFormat;
        }
        
        private void UpdateTextBox(string matchedText, string newText)
        {
            this.EditableTextBoxSite.Text = matchedText;
            this.EditableTextBoxSite.SelectionStart = newText.Length;
            this.EditableTextBoxSite.SelectionLength = matchedText.Length - newText.Length;
        }